The present invention relates to an arrangement for coupling electromagnetic waves into and/or out of a microwave device which comprises at least one dielectric resonator. The dielectric resonator comprises a non-linear dielectric substrate with a high dielectric constant and coupling is provided through coupling loops.
Still further the invention relates to a method of coupling microwave signals into and/or out of a microwave device including at least one dielectric resonator with a non-linear dielectric substrate having a high dielectric constant.
Dielectric and parallel-plate resonators and filters for microwave frequencies using dielectric disks of any shape, for example circular, are known, see for example Vendik et. al., El. Lett., vol. 31, P. 654, 1995, which herewith is incorporated herein by reference. Parallel-plate resonators comprising a non-linear dielectric material with extremely high dielectric constants, for example ferroelectric materials or an antiferroelectric material, have small dimensions and can be used to provide very compact filters in the frequency band of 0.5-3.0 GHz which is the frequency band in which most advanced microwave communication systems operate today. Such non-linear dielectric materials may for example be STO (Strontium Titanate) which has a dielectric constant of about 2000 at the temperature of liquid nitrogen and a dielectric constant of about 300K at room temperature. As an example, the resonant frequencies of circular STO parallel-plate disk resonators having a diameter of 10 mm and a thickness of 0.5 mm are in the range of 0.2-2.0 GHz depending on the temperature and on the applied DC biasing. At these frequencies the wavelengths of the microwave signals are in the range of about 15-150 cm which is much larger than the dimensions of the resonator itself.
It is known how to excite dielectric and parallel-plate resonators by simple probes or loops. In most practical cases the thickness of a parallel-plate resonator is much smaller than the microwave wavelength in order for the resonator to support only the lowest is order TM-modes and in order to keep the DC voltages, which are required for the electrical tuning of the resonators with nonlinear dielectric fillings, as low as possible. This is discussed in Gevorgian et al., "Low Order Modes of YBCO/STO/YBCO Circular Disk Resonators" IEEE Trans. Microwave Theory and Techniques, Vol. 44, No. 10, October 1996. This document is also incorporated herein by reference.
However, some microwave devices, such as for example passband filters, often require strong (i.e. near-critical or over-critical) input/output couplings. To achieve such strong couplings in resonators or devices based on thin parallel-plate disk resonators, particularly having an extremely high dielectric constant such as STO, it is practically impossible to use known coupling arrangements such as loop or probe couplers, for example as discussed in Kajfez, Guillon: Dielectric resonators, 1990, chapter 8, and e.g. page 282, chapter 6.6.
Probe coupling, which is a coupling mainly to the electrical field, is not efficient since almost all the microwave power is reflected from the walls of the resonator. Because of the extremely high dielectric constant of for example STO, the walls of the resonator serve as near perfect magnetic walls with reflection coefficients close to 1 which follows from a simple relationship: EQU .GAMMA.=(.epsilon.-1)/(.epsilon.+1)
.GAMMA. being the reflection coefficient and E being the dielectric constant.
Furthermore, known loop couplings (coupling to the magnetic field) are also not efficient. In a thin parallel-plate resonator with only TM-modes, the magnetic field lines are parallel to the plates of the resonator. Because of the small thickness of the resonator only a small amount of the magnetic field lines of the external traditional coupling loop is matched to the magnetic field lines inside the resonator and the matching cannot be increased by making the area of the coupling loop larger.
T. Hayashi et. al., "Coupling structures for superconducting disk resonators, Electronic Letters", Vol. 30, No. 17, pp. 1424-1425, 1994, has suggested an enhanced capacitance coupling arrangement to achieve a strong input/output coupling in filters based on microstrip parallel-plate resonators. This arrangement is however only effective for dielectric resonators in which the dielectric has a low dielectric constant, approximately between 10-20. Such resonators are much too large for a number of applications Still further it is only effective for the fundamental TM 110-mode.
K. Bethe, "Uber Das Mikrowellenverhalten Nichtlinearer Dielektrika", Philips Res. Reports, Suppl. 1970, No. 2, p. 44 shows rectangular waveguides for TM 110-mode input/output couplings for high dielectric constant parallel-plate resonators, for example of STO. However, the coupling arrangement is bulky and not at all suitable for small size applications An additional DC-biasing arrangement is required which is disadvantageous since it introduces reactances into the microwave circuit which results in a degradation and reduction of the quality factor and of the overall.
Vendik et. al., Electronic Letters, Vol. 31, p. 654, 1995 discloses a coaxial waveguide for TM 020-mode input/output couplings for a resonator comprising a substrate with a high dielectric constant. The coupling is then applied through the central rod of a coaxial line. For tuning purposes external bias tees are used. The coupling arrangement of this device is bulky and also not appropriate for small resonators or small devices in general.
Furthermore the biasing arrangement also introduces reactances into the microwave circuit resulting in a performance degradation. High dielectric constant parallel-plate resonators, for example comprising diaelectrics of STO, have a high mode density This makes the use of traditional probe and loop coupling arrangements disadvantageous since they provide approximately the same coupling for all modes. In a number of cases only one mode should be excited. In for example narrow band filters only one mode is desired while the other modes create spurious transmissions in the rejection band and hence degrades the overall performance of the filter. To avoid this problem mode selective input/output coupling arrangements are needed.
Another disadvantage of the known arrangements is that electrically tunable parallel-plate resonators based on non-linear dielectrics, such as for example STO, require external DC biasing (in the form of ohmic contacts to the metallic plates of the resonator) in order to control the resonant frequency. According to the Swedish Patent Applications, by the same applicant, 9502138-2 and 9502137-4, (corresponding to U.S. patent application Ser. No. 08/985,149, which has been allowed, and Ser. No. 08/989,166, respectively) DC biasing is provided through introduction of an additional arrangement into the resonator design. Such an arrangement however affects the resonant frequency and furthermore it may deteriorate the quality factor (Q) of the resonator.
Finally a number of resonators are known which are based on ferromagnetic resonances. The resonant frequency is then determined by the microscopic properties of the materials used such as ferromagnetic resonance, anti-ferromagnetic resonance, electronic paramagnetic resonance etc. (and the dimension of the resonator is not given by the frequency of the wavelength of the microwave signal). In such resonators the lowest resonant frequency is limited by material properties, and the size of the material used in the resonator is usually made arbitrary small and not related to the wavelength of the microwave signal. The magnetic coupling loops used for such resonators are designed so as to provide a uniform magnetic field distribution in the ferrite. A mode selection is then not possible. An example of such a filter with the associated coupling arrangements is for example shown in U.S. Pat. No. 4,197,517. Also U.S. Pat. No. 4,945,324 shows an example on such a magnetic filter.